1. Field of the Invention
The present application relates to telecommunication equipment employing SCR type hold circuits and more particularly a special SCR snubber circuit for preventing false hold "turn-on" in response to a static discharge.
2. Background Art
Since the development of telephone circuitry employing active components, i.e., solid state devices, etc., the addition of many features have been easy to add to the telephone. One such feature has been the addition of a so-called "hold" circuit. With the provision of such circuitry the telephone call may be electronically latched unto an artificial load allowing the hand set to return to the hook switch, until the call is picked up on an extension phone or again at the original telephone, at which time the hold condition is dropped automatically.
Previous hold circuits frequently made use of expensive components such as latching relays and/or integrated circuits comparators and related associated circuitry adding substantial cost to the telephone. In many instances such circuitry has poor sensitivity because of the compromise between trigger "on" sensitivity and the ability to drop completely on high resistance extensions and in the presence of low central office battery voltage. Some hold circuit methods require the use of a microprocessor and related circuitry to sense the line voltage change with conversion to a digital signal. It is obvious that all such circuitry suffers to some degree from overcomplexity with reduction in the intended reliability that stems from the increased number of components required in such circuit designs.
Many of the existing hold circuits also suffer from the lack of inclusion of an automatic delay or time-out facility. In this event, for example, if a call is placed on "hold" and an extension of the original phone is not again taken off hook within a predetermined time period, it is desirable that the "hold" condition should be dropped, to avoid excessive charge against the telephone subscriber. Another hazard which occurs and is particularly undesirable with the no time-out hold circuits is the effect of static discharge from a person to the telephone. Such static discharge can trigger the hold circuit into the "on-hold" state and will remain on until either the handset is picked up or a disconnect signal is received. In the event that the telephone is located on an ungrounded metal desk, a discharge to desk can also trigger the hold circuit to an "on" state.
Those hold circuits that are most susceptible to the static discharge effect are circuits that employ silicon controlled rectifier (SCR) devices. Such SCR circuits have the advantage of simplicity and high sensitivity for triggering on long telephone loops and usually employ what is termed a "sensitive gate" SCR. Such sensitive gate SCR's are also sensitive to a rapid rate of change of anode voltage that can trigger the SCR into the conductive mode or "on" state thereby turning on the hold circuit. A higher voltage rating for the SCR helps to overcome the problem but also makes it much more expensive. Without special protection or snubbing circuitry the anode of the SCR can be exposed to a rate of change of voltage and high absolute voltage far in excess of its ability of even the best SCR's to resist.
With a static discharge present, a very fast steep wave front pulse of up to 25,000 volts may be applied to the circuit. This can result in a very high voltage on the anode of the SCR and even very high frequency oscilation in the circuit which increases the probability of SCR turn on. In the case of the telephone on a metal desk, the desk can also become charged developing a corresponding charge in the ground plane of the telephone printed circuit board which draws its charge rapidly to the SCR to turn it on. Addition of shields to the telephone may help under some conditions but not under certain other conditions; therefore, a circuit to control the SCR under the discharge conditions is needed.
Some prior art snubber circuits have consisted of one or more transistors. The transistor circuits are costly and may not be effective with a sufficiently fast discharge. Transistors may also have to be of the same high voltage rating as the SCR, thus increasing the cost. Some have attempted to replace the SCR wih an NPN and PNP transistor combination but the problem with this is that the high voltage transistors (700 volt ratings) are hard to obtain and very costly, especially if of the PNP type. The high voltage ratings are necessary to withstand the lighting surge invironment particularly associated with telephone instruments.
Other snubber circuits have used combinations of resistors, diodes, capacitors and inductors to bridge the SCR's anode to cathode. All the circuits found in the literature suffer due to significant unbridged resistance in the snubber circuit. Computation shows that even as little as one ohm of unbridged resistance (such as that in a forward biased diode) is enough to dramatically raise the instantaneous voltage rate of change on a static discharge at the anode of the SCR. Another problem present in some of the prior art snubber circuits is the use of a capacitor with no protection for it due to over voltage and no discharge path for it so that once charged it will stay charged for a long period and not function well in the case of a second static pulse. Finally, in the case of the "hold" circuit no significant bridging loss can be tolerated when the "hold" circuit is not on. Such requirements as minimum voice band impedance on-hook, ring signalling impedance and off-hook impedance must be maintained. Also audio loss in the speech circuit because of the use of a snubber circuit must be less than 0.5 db. to be acceptable, or zero loss if possible.